Method of joining waterproof sheets, and their joint structure

ABSTRACT

A method of joining narrow waterproof sheets to obtain a waterproof sheet of wide width having a gently curved joint which is free from any acutely stepped portion. It comprises disposing a cure tape between the overlapping edge portions of the sheets to be joined, positioning a piece of cushiony material below or above the overlapping edge portions, and heating the overlapping edge portions and the cure tape under pressure. The cure tape has a width which is substantially equal to that of the overlapping edge portions before pressure is applied thereto. When pressure is applied, the cure tape projects in opposite directions beyond the extremities of the overlapping edge portions. One form of the joint thus formed is flat on one surface and gently curved on the other surface, while another form is gently curved on both surfaces thereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of joining waterproof rubber sheetsfor a reservoir, irrigation pond, pond for disposal of industrial waste,irrigation channel, canal, roof of a house, or the like. Moreparticularly, it is concerned with a method which includes disposing acure tape between the overlapping edge portions of a pair of vulcanizedrubber base sheets having a relatively small width, applying a piece ofcushiony material to one of the sheets, and heating the overlapping edgeportions under pressure to thereby provide a waterproof sheet of widewidth having a joint which is in the form of a gently curved wave incross section. This invention is also concerned with the construction ofany such joint in a waterproof sheet.

2. Description of the Prior Art

Concrete or asphalt has been mainly used for the watertight lining of areservoir or the like for a long time. Recently, however, rubber sheet,or resin films have come to be widely used for that purpose, since theyare higher in flexibility, weatherability, and adaptability to anymovement in the base to which they have been applied, easier to apply,and lower in cost.

These rubber sheets or resin films are available in lengths of 200 to300 m, but the manufacturing facilities restrict their width to usuallya range of 1 to 3 m. A multiplicity of sheets having such a small widthare transported to the site of construction, and joined together alongtheir edges manually to make a sufficiently large waterproof sheet.There are, however, so many joints to be made that the efficiency of thework is extremely low. As the work is usually done outdoors, it isseriously affected by the weather and environmental conditions.Moisture, water drops and dust have a very adverse effect on thestrength of the joints. Nowadays, a reservoir, an irrigation pond, orthe like is very large. It is necessary to make unit sheets of widewidth available at the site of construction in order to minimize thejoints to be made at the site, and shorten the time required for joiningthe sheets.

A number of methods are known for obtaining unit sheets of wide width.They are, for example, (1) overlapping unvulcanized rubber sheets oneach other along their edge portions of predetermined width, andvulcanizing them, (2) overlapping vulcanized rubber sheets on each otheralong their edge portions, disposing between their overlapping edgeportions a cure tape which will be vulcanized under heat and obtainincreased strength, and heating their overlapping edge portions and thecure tape therebetween, or (3) overlapping vulcanized sheets on eachother along their edge portions of predetermined width, and joiningtheir overlapping edge portions with an adhesive. According to any ofthese methods, however, the sheets to be joined are overlapped on eachother, and there is formed a stepped joint. When waterproof sheets ofwide width obtained as hereinabove described are transported to the siteof construction, and joined together, the stepped joints in one sheetfail to make tight connection with another sheet, and leave clearancesthrough which water may leak.

The difficulty hereinabove pointed out will be explained morespecifically with reference to FIGS. 1 to 3 of the drawings. Narrowsheets 1 and 2 are joined to each other along their overlapping edgeportions to provide a sheet of wide width as shown in FIG. 1. This sheetof wide width has a stepped joint. It is joined at the site ofconstruction to another sheet 3 of wide width in a T-shaped pattern asshown in FIG. 2, though they can, of course, be joined in any otherpattern, too. A clearance C is formed between the stepped joint and thesheet 3 as shown in FIG. 3, and may cause water leakage therethrough. Inorder to prevent any such water leakage, it has been usual to apply asealing material, or unvulcanized tape 4 to the stepped joint beforeplacing the sheet 3 as shown in FIG. 4. This method is, however, verycumbersome. Moreover, it is difficult to apply the sealing materialuniformly under the bad conditions prevailing at the site ofconstruction. Water leakage still occurs often, and is the gravestproblem in the watertight lining of a reservoir or the like employingwaterproof sheets.

SUMMARY OF THE INVENTION

It is an object of this invention to eliminate the drawbacks of theprior art, and provide a method which makes it possible to joinwaterproof sheets easily and efficiently, and construct a waterproofsheet assembly which is free from any water leakage. It is anotherobject of this invention to provide an improved watertight joint for awaterproof sheet assembly. According to this invention, a cure tape isdisposed between the overlapping edge portions of every adjoining pairof narrow vulcanized or unvulcanized rubber sheets, a piece of cushionymaterial is applied to one of the sheets, and the overlapping edgeportions thereof and the cure tape are heated under pressure, wherebythere is formed a waterproof sheet of wide width having a gently curvedjoint which is free from any acutely stepped portion. Such sheets ofwide width are joined at the site of construction to provide awaterproof sheet assembly having perfectly watertight joints therein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of a pair of narrow sheets joinedtogether by a method known in the art;

FIG. 2 is a top plan view illustrating a pair of sheets of greater widthobtained by the method shown in FIG. 1, and joined together;

FIG. 3 is a sectional view taken along the line A--A' of FIG. 2;

FIG. 4 is a side elevational view of the two narrow sheets joinedtogether by the known method, and having a sealing material to theirjoint;

FIG. 5 is a side elevational view illustrating a method of joining apair of narrow sheets together according to this invention;

FIG. 6 is a side elevational view showing the two narrow waterproofsheets having their overlapping edge portions heated under pressure inaccordance with the method of this invention;

FIG. 7 is a side elevational view showing a joined formed between thewaterproof sheets according to the method of this invention;

FIGS. 8 and 9 are side elevational views showing modified forms ofjoints formed in waterproof sheets according to this invention; and

FIG. 10 is a side elevational view showing a testpiece prepared for ashearing strength test by joining a pair of narrow waterproof sheetsaccording to the known method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIG. 5 of the drawings, there is illustrated a methodembodying this invention. A cure tape 5 is disposed between theoverlapping edge portions of a pair of vulcanized waterproof sheets 1and 2. The cure tape 5 has a width which is equal to the overlappingwidth of the sheets 1 and 2. The overlapping edge portions and the curetape 5 disposed therebetween are positioned between upper and lowerpress members P₁ and P₂. A piece of cushiony material 6 is placed on thelower press member P₂. The overlapping edge portions of the sheets 1 and2, and the cure tape 5 are heated under pressure. The upper press memberP₁ has a flat pressing surface 7, and the flat cushiony material 6 onthe lower press member P₂ defines a flat pressing surface 8, too. Thetwo pressing surfaces 7 and 8 are in parallel to each other. Unless thepressing surfaces are in parallel to each other, the molten cure tapedoes not flow uniformly, but an acutely stepped joint results. Thepressing surfaces 7 and 8 must be in parallel to each other, though theymay be curved instead of being flat.

The overlapping portions of the sheets 1 and 2 are heated under pressureas shown in FIG. 6. The cushiony material, or rubber cushion 6 on thelower press member P₂ is deformed to define a gently curved concavity inaccordance with the distribution of pressure thereon. The molten curetape 5 flows out transversely beyond the edges 12 and 13 of the sheets,whereby the two sheets 1 and 2 are joined together with a gently curvedjoint as shown in FIG. 7. The joint has a gently curved lower surface10, and a flat upper surface 11 as shown in FIG. 7. The gently curvedlower surface 10 is formed by the molten cure tape 5 flowing outtransversely beyond the edge 12 of the sheet 2 with the aid of thecushiony material 6, and is free from any acutely stepped portion. Theupper surface 11 of the joint is flat, since there is no cushionymaterial on the pressing surface 7 of the upper press member P₁. Theflat upper surface 11 includes a smaller area defined by the molten curetape 5 which has flowed out beyond the edge 13 of the sheet 1.

The fluidity of the cure tape depends on its viscosity and thickness,the hardness and thickness of the cushiony material, and the amount ofthe pressure to be applied for joining the sheets. According to thisinvention, the cure tape comprises a tape of vulcanizable, butunvulcanized rubber, such as an ethylene-propylene terpolymer (EPT)containing dicyclopentadiene as the third component and having an iodinevalue of 20, or containing ethylidene norbornene and having an iodinevalue of 24, isobutylene-isoprene rubber, that is, butyl rubber (IIP),or mixture of EPT and IIR. The cure tape further contains a vulcanizingagent, a vulcanization accelerator and a softening agent, and stearicacid, carbon black, an inorganic filler, zinc oxide, etc. if required.

The cure tape is fluidizable if it has a Mooney viscosity [ML₁₊₅ (100°C.)] not exceeding 80, depending on the amount of pressure to beapplied, and the nature of the cushiony material 6. In order to obtain asmooth flow of the cure tape in an area M in FIG. 7, however, it isdesirable for the cure tape to have a Mooney viscosity of, say, 15 to45. The cure tape 5 must be greater in thickness than the sheets 1 and 2to be joined. In order to provide a smooth flow, it should be about 1.1to 2.0 times greater in thickness than the sheets 1 and 2. The use of athinner cure tape would result in formation of a stepped portion at N inFIG. 7.

In order to obtain a smooth finish on the lower surface of the joint, itis most important to select the appropriate cushiony material 6.Although the appropriate cushiony material 6 depends on the viscosity ofthe cure tape 5, and the amount of pressure to be applied, it is usuallydesirable to employ a cushiony material having a lower degree ofhardness than the sheets to be joined. Examples of the appropriatematerial include a sheet of rubber having a hardness of 30° to 70°(JIS), particularly heat resistant IIR or EPT, or a mixture of IIR andEPT, or a nonwoven fabric, or a foam.

The pressure to be applied to the overlapping porrions of the sheets 1and 2, and the cure tape 5 may be of any level unless the cure tapefoams. Although it is preferable for the cure tape to have a width whichis equal to the overlapping width of the vulcanized or unvulcanizedsheets to be joined, it may also be somewhat smaller or larger than theoverlapping width.

The waterproof sheets to be joined in accordance with this invention arevulcanized or unvulcanized sheets composed mainly of EPT, IIR, or amixture of EPT and IIR.

According to the method shown in FIGS. 5 and 6, the cushiony material 6is simply placed on the lower press member P₂, and not secured thereto.It is also possible to dispose the cushiony material 6 above thewaterproof sheets to be joined. If in this case, the cushiony material 6is secured to the upper press member P₁, however, the exchange of thecushiony material 6 is a very troublesome job. If the cushiony material6 is positioned below the sheets to be joined, it does not need to besecured to the lower press member P₂ by bolts, or the like, but can bechanged very easily when required. In either event, however, it ispossible to obtain a smoothly finished joint as shown in FIG. 7.

It is conceivable to employ two pieces of cushiony material on theopposite sides, respectively, of the sheets to be joined. This method isnot recommendable, since it has a number of disadvantages. A greateramount of pressure has to be applied. The low thermal conductivity ofthe cushiony material brings about a low thermal efficiency. A greateramount of work is required for changing the cushiony material.

According to this invention, the overlapping portions of the sheets tobe joined, and the cure tape disposed therebetween are heated at atemperature of 140° C. to 160° C. under a pressure of 3 to 4 kg/cm² for5 to 15 minutes.

Referring now to FIGS. 8 and 9, there are shown modified forms of thejoint in the waterproof sheets. The joint shown in FIG. 8 is obtained bythe cushiony material disposed both above and below the waterproofsheets to be joined. The cure tape 5 projects to an equal extent fromboth of the ends 12 and 13 of the waterproof sheets 1 and 2, and definesthe upper and lower surfaces 11 and 10 of the joint which are bothgently curved, but free from any acute corner. The joint shown in FIG. 9is identical to that shown in FIG. 7, except that the overlapping edgeportions of the waterproof sheets 1 and 2 are both of the steppedconfiguration.

According to the method of this invention, it is possible to joinwaterproof sheets without forming any acutely stepped portiontherebetween, form a joint of improved strength therebetween, andconstruct a waterproof sheet assembly more easily and efficiently at thesite of construction.

A joint having a clearance N therein was formed by a method known in theart as shown in FIG. 10, and tested for shearing strength. It was brokenearlier than the sheets per se, and found lower in strength than thesheets. This was due to the localized concentration of stress in theclearance N. No such concentration of stress was, however, observed onthe joint formed according to this invention, since it was smooth, andfree from any acutely stepped portion. The results of joint efficiencytests also indicated that the joint formed according to this inventionwas comparable in strength to the sheets per se. The joint was notbroken, but it was the sheets that were broken.

The invention will now be described with reference to examples.

EXAMPLE 1

A joint was formed from the following materials under the followingconditions.

Sheets to be joined: Vulcanized sheets of a mixture ofethylene-propylene terpolymer and butyl rubber in a ratio by weight of7:3, each having a thickness of 1.5 mm, and having an overlapping widthof 20 mm.

Cure tape:

    ______________________________________                                        Composition (parts by weight)                                                 ______________________________________                                        EPT containing dicyclopentadiene as the                                                                70                                                   third component, and having an iodine                                         value of 20                                                                   IIR                      30                                                   ZnO                      5                                                    SRF carbon               40                                                   Escoretz (product of Esso Chemical)                                                                    15                                                   Paraffinic process oil (product of Maruzen                                                             20                                                   Petroleum)                                                                    Sulfur                   1.5                                                  Dipentamethylenethiuram tetrasulfide                                                                   5.5                                                  ______________________________________                                    

Mooney viscosity [ML₁₊₅ (100° C.)]: 25

This unvulcanized rubber composition was formed into a sheet having athickness of 2.0 mm and a width of 20 mm, and disposed between thevulcanized sheets to be joined.

Cushiony material: A 7.0 mm thick sheet of butyl rubber having ahardness of 55° (JIS).

Pressure: 3.5 kg/cm².

Vulcanizing conditions: At 158° C. for 10 minutes.

The following results were obtained:

Overlapping joint thickness: 3.7 mm

Extended cure tape width: 50 mm

Joint efficiency: The joint was not broken, but it was the sheets thatwere broken.

Leak tests: The joint formed as shown in FIG. 2 was tested for waterleakage in the absence of any sealing material therein. The tests wereconducted at a water pressure of 1.0 kg/cm² for eight hours, and 0.4kg/cm² for eight days. No leakage occurred in either case.

It has, thus, been found that the joints formed according to thisinvention are entirely leak-free without the aid of any sealingmaterial, or unvulcanized tape, or the like.

EXAMPLE 2

A pair of unvulcanized sheets each having a thickness of 1.5 mm, a widthof 1.2 m and a length of about 8 m were overlapped along their adjoiningedge portions, and a cure tape was disposed between their overlappingedge portions. They were placed on a molding table, and pressure wasapplied to the overlapping edge portions by a hand roll, whereby thesheets were joined together to form an unvulcanized sheet of wide width.This sheet was wound about a drum having a width of about 8 m. When itwas wound on the drum, a canvas called a "vulcanizing liner" was woundtogether to separate layers of the unvulcanized sheet and prevent theiradhesion to one another. This drum was placed in a vulcanizer, and steamvulcanization was carried out for a period of 60 minutes at a pressureof 5 kg/cm². The joint, and simultaneously the sheet per se were, thus,vulcanized, whereby a waterproof sheet of wide width was obtained. Thejoint was tested for shearing strength. It was not broken, but it wasthe sheet per se that was broken, as had been the case with the jointbetween the vulcanized sheets.

In view of the foregoing, according to the present invention, waterleakage at the joint portion of the sheets can be prevented, and thejoint portion provides sufficient mechanical strength. Therefore, sheetinstallation workability becomes remarkably improved.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. A method for joining edges of two sheetstogether, comprising: disposing a cure tape, which will flow under heatand pressure and is heat curable, between overlapping edge portions ofthe sheets to be joined, said tape having a width which is substantiallyequal to that of said overlapping edge portions, positioning said edgeportions with said cure tape between pressing members with a resilientcushion between one of said sheets and one of said members, applyingheat and pressure to said sheets and the interposed cure tape causingsaid cure tape to flow, thereby resulting a gently curved joint betweensaid two sheets free from any acutely stepped portion, said cushionbeing deformed when said pressure is applied so as to form a shapingcavity for the joint therein.
 2. A method as set forth in claim 1,wherein said resilient cushion is positioned under said sheets.
 3. Amethod as set forth in claim 2, wherein said heating under pressure iscarried out by a pair of vertically spaced apart press members of whichthe upper defines an upper flat pressing surface, said cushiony materialbeing flat and carried on the lower of said press members, and defininga lower flat pressing surface which is in parallel to said upperpressing surface, said sheets to be joined being heated under pressurebetween said upper and lower pressing surfaces.
 4. A method as set forthin claim 1, 2 or 3, wherein said piece of resilient cushion is a sheetof rubber.
 5. A method as set forth in claim 4, wherein said sheet ofrubber comprises butyl rubber having a hardness of 30° to 70° (JIS). 6.A method as set forth in claim 1, wherein said cure tape comprises anunvulcanized sheet of a vulcanizable rubber composition consistingmainly of a rubbery polymer selected from the group consisting of anethylene-propylene terpolymer, butyl rubber, and a mixture of anethylene-propylene terpolymer and butyl rubber, said composition havinga Mooney viscosity [ML₁₊₅ (100° C.)] of 15 to
 45. 7. A method as setforth in claim 1, wherein said cure tape is 1.1 to 2.0 times greater inthickness than said sheets to be joined.
 8. A method as set forth inclaim 1, wherein each of said sheets to be joined is a sheet ofvulcanized rubber selected from the group consisting of anethylene-propylene terpolymer, butyl rubber, and a mixture of anethylene-propylene terpolymer and butyl rubber.